Ion-exchange resins prepared by copolymerizing acrylic and methacrylic acids with crosslinking monomers are well known, and are useful as weakly acidic cation exchangers. Because the monomers of these acids are hydrophilic, and water soluble to a significant extent, the copolymers are more difficult to prepare by suspension polymerization than resins made from acrylic and methacrylic esters. The esters are relatively insoluble in water and hydrophobic, and as a result polymerize within the droplets of the oil phase, forming relatively clean beads. The acids, however, tend to polymerize partially within the aqueous suspending phase as well, forming undesirable, aqueous-phase polymer that wastes monomer and interferes with formation of clean copolymer beads.
Special techniques must be employed to minimize this tendency of the acid monomers to dissolve in the aqueous phase. One such technique is to add inorganic salts to the aqueous phase, "salting out" the monomer, or reducing its solubility in the resulting brine to the point where it is forced into the oil phase where its polymerization is desired. Not only must the salt be recovered, either for re-use or to permit ecologically responsible disposal of the waste aqueous phase, but it tends to cause the oil-phase droplets to distort during polymerization. This promotes formation of non-spherical copolymer beads, which have unpredictable hydraulic properties and pack poorly in ion-exchange columns.
Porous resin beads are particularly advantageous for certain uses, for example as adsorbents or as ion exchangers with rapid kinetic properties. Porosity is introduced into the beads in a number of different ways; the use of a phase-separating agent to form macroreticular beads, as disclosed by Meitzner et al., in U.S. Pat. No. 4,221,871 is one common way to generate this porosity. Selection of the phase-separating agent offers some control over the total porosity, the pore sizes and the pore-size distribution in the finished beads. Because methacrylic acid in the oil phase dissolves some water from the aqueous phase, the water reduces the solubility of the total monomer solution in the oil phase, and phase separation occurs in the absence of an introduced phase-separating agent. Consequently the preparation of macroreticular resins from methacrylic acid is simpler, but control over the pore parameters is limited.
Anhydrides of acids are known to hydrolyze in the presence of strong acids or bases to form the acids themselves. Anhydride polymers, such as polymers of maleic anhydride, introduce into copolymers reactive sites that may be used for subsequent reactions, such as ion-exchange functionalization, as is disclosed in U.S. Pat. No. 2,988,541 to Semon et al. or U.S. Pat. No. 3,871,964 to Huper et al. Methacrylic anhydride has been disclosed, within a broad disclosure of esters, amides and nitriles of methacrylic acid, as a monomer for preparing copolymers that could then be functionalized as a weakly acidic cation exchanger (U.S. Pat. No. 2,324,935 to Kautter), and British Patent No. 894,391 to Bayer suggests methacrylic anhydride as one of a broad range of olefinically unsaturated carboxylic acids, esters and anhydrides from which copolymer having "a sponge structure" might be made, but no practical preparations of such copolymers are shown.
Others, as for example Kraemer et al. in U.S. Pat. No. 4,070,348, Lehmann et al. in U.S. Pat. No. 3,764,477 and Barnes in U.S. Pat. No. 2,308,581, made copolymers of methacrylic anhydride by bulk or precipitation polymerization or reverse-phase, suspension polymerization in which the continuous, suspending phase was an organic rather than an aqueous liquid. Lehmann et al. (above) mentions suspension polymerization as a possibility, but without exemplifying a practical preparation.